Alpenglow

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alpenglow

The occasional reappearance of sunset colors on a (snow-covered) mountaintop soon after sunset and a similar phenomenon before sunrise.

Alpenglow has three phases. During evening twilight, the first stage is the mountain peak's usual coloration seen at low sun elevations h0 (h0 < 2°). Second is the alpenglow proper that occurs a few minutes after the first color has faded (h0 slightly less than 0°). The peaks are still in direct sunlight, and their colors are purer and often pinker than before. The alpenglow boundary may first occur hundreds of meters below the summit, then moves upward, and finally fades as the atmosphere's dark segment rises. Third is the afterglow, which occurs nearly simultaneously with the first purple light. The peaks are no longer in direct sunlight; the illumination is more diffuse and its boundary vaguer than in the earlier stages. The third stage lasts longer than the other two (-5° < h0 < -9°), and its color varies from yellow to purple. A faint second afterglow has been reported and is associated with the rare occurrence of a second purple light. The alpenglow appears to be much less common at sunrise than at sunset. The morning colors are more pink and purple, while those of evening are more orange and red.

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